Such an optical pointing device is already known in the art. International Patent Application No WO 03/049018, filed in the name of the same Assignee, which is incorporated herein by reference, discloses a method as well as a device for motion detection in an optical sensing device, such as an optical mouse.
FIG. 1 is a generalized schematic bloc diagram of an optical pointing device in accordance with the prior art. It comprises a photodetector array 420 including a plurality of pixels, this photodetector array 420 being coupled to processing means 400 (or motion detection processing circuit) for processing the signals outputted by the photodetector array 420.
A comparator array 415 may be interposed between processing means 400 and array 420, this comparator array 415 including a plurality of comparator circuits each for comparing the light intensity of a first pixel of array 420 with the light intensity of a second pixel of array 420 and for outputting a resulting edge direction condition.
A distinction is made between edges according to their “direction”. In particular, one defines two distinct edge direction conditions: (i) a first edge condition, or positive edge, defined as a condition wherein the light intensity of a first pixel is less than the light intensity of a second pixel; and (ii) a second edge condition, or negative edge, defined as a condition wherein the light intensity of the first pixel is greater than the light intensity of the second pixel.
Taking photodetector array 420 as an example, a first axis, namely axis X, extends with a positive direction from left to right and a second axis, namely axis Y, extends with a positive direction from bottom to top. Accordingly, a positive edge will be defined between a selected pixel and a pixel on its right if the detected light intensity of the selected pixel is less than the light intensity of the pixel on its right. Conversely, and taking the example of two pixels aligned along axis Y, a negative edge will be defined between the selected pixel and a pixel on its upper side if the light intensity of the selected pixel is greater than the light intensity of the upper pixel. Both compared pixels can be adjacent or non-adjacent.
The optical pointing device further comprises at least one light source 410 such as a LED, which produces radiation, that impinges on a portion of a surface S. Surface S may be a planar or non-planar surface, such as a surface over which the pointing device is moved (as in the case of an optical mouse), the surface of a ball (as in the case of an optical trackball) or any other suitable surface that may provide an appropriate intensity pattern for detection by photodetector array 420. It should be mentioned that a light source is not, strictly speaking, necessary and that ambient light reflected by surface S may directly be used.
Processing means 400 is further adapted to communicate in a bi-directional manner with an interface 450 that communicates in turn with a host system (not illustrated) over a bus 455. Cursor control signals (and eventually other signals related to the optical pointing device) are supplied to the host system over bus 455. Processing means 400 may also receive information, such as configuration signals, over bus 455 from the host system.
Processing means 400 is essentially designed to intermittently sample the pixel outputs of photodetector array 420 in accordance with a defined sequence. The edge information of two successive samples is compared and a relative motion measurement is extracted by processing means 400. The adequate cursor control signals are then derived from the relative motion measurement and transmitted to the host system via line interface 450.
Still referring to the International Patent Application No WO 03/049018, it is disclosed a so-called “Peak/Null Motion Detection” algorithm. Each row and column of the photodetector array is further analysed to find specific inflection conditions (hereinafter defined as a first inflection condition, or “peak”, and a second inflection condition, or “null”) in the direction of successive edges along a selected axis (in practice along both the X and Y axes). As illustrated in FIG. 2, the first inflection condition, or peak, is defined as the succession, along a determined axis (X or Y), of a positive edge (arrow pointing upwards in FIG. 2) followed by a negative edge (arrow pointing downwards in FIG. 2). Similarly, the second inflection condition, or null, is defined as the succession, along the determined axis, of a negative edge followed by a positive edge.
In contrast to the above edge direction conditions, it will be appreciated that the edge inflection conditions do not appear everywhere. Strictly speaking, one should also consider that there exists a third inflection condition representative of the fact that there does not exist, at a selected location, any inflection in the direction of the edges.
With reference to FIG. 3, one will now describe the basic principle for detecting motion based on the above edge inflection data. FIG. 3 schematically shows an exemplary situation (in case of single axis motion detection) of a succession of ten successive edge direction conditions and extracted edge inflection conditions determined from two successive flashes (i.e. conditions derived from two successive light intensity patterns detected by the photodetector array). This exemplary situation is typical of sub-pixel motion (displacement of less than the pixel pitch between two successive flashes). As shown in FIG. 3, three edge inflection conditions are detected, namely a peak between the second and third edge direction conditions, a null between the fourth and fifth edge direction conditions, and a peak between the seventh and eighth edge direction conditions. Following the second flash, one can see that only the null condition moved one unit to the right (this situation again being typical of sub-pixel motion). In addition to accumulators for tracking motion of edge inflection conditions, one additional accumulator per axis is provided for counting the total number of edge inflection conditions (peaks and nulls together) appearing on each axis.
Referring again to the exemplary situation of FIG. 3, one will understand that the accumulator associated to the tracking of motion of edge inflection conditions would be incremented, the total number of peaks and nulls detected by accumulator associated for counting the total number of edge inflection conditions being in this case three. In case of motion detection along two axes (as in the case of an optical pointing device), one will of course have understood that these steps are performed for each row of the array along axis X and each column of the array along axis Y.
A calculation method may consist in computing the displacement values along axes X and Y directly, as summarized by the following analytical expressions:
                              X          DISPLACEMENT                =                                            (                                                N                                      PEAK                    -                    RIGHT                                                  +                                  N                                      NULL                    -                    RIGHT                                                              )                        -                          (                                                N                                      PEAK                    -                    LEFT                                                  +                                  N                                      NULL                    -                    LEFT                                                              )                                            (                                          N                XPEAK                            +                              N                XNULL                                      )                                              (        1        )            
                              Y          DISPLACEMENT                =                                            (                                                N                                      PEAK                    -                    UP                                                  +                                  N                                      NULL                    -                    UP                                                              )                        -                          (                                                N                                      PEAK                    -                    DOWN                                                  +                                  N                                      NULL                    -                    DOWN                                                              )                                            (                                          N                YPEAK                            +                              N                YNULL                                      )                                              (        2        )            
This method requires a minimum of two accumulators per axis, one for tracking motion of edge inflection conditions (peaks and nulls being still tracked independently) and another one for tracking the total number of edge inflection conditions detected along the selected axis.
Although this calculation method gives acceptable results in most situations, it has been shown within the scope of the present invention that the detected motion is not as reliable as it could be in some critical situations. In fact, when the optical sensor is illuminated along one axis (e.g. along axis Y), while the detected motion with the above described calculation method is close to the real motion along the non-illuminated axis (i.e. axis X), conversely, the motion detected along the illuminated axis is much less reliable. In the case of illumination in between both axes, the detected motion along both axes becomes less reliable.